    (function(){
        function w() {
            var r = document.documentElement; //根元素html
            var a = r.getBoundingClientRect().width;
                if (a > 750 ){
                    a = 750;
                } 
                rem = a / 7.5;
                r.style.fontSize = rem + "px";
        }
        var t;
        w();
        window.addEventListener("resize", function() {
            clearTimeout(t);
            t = setTimeout(w, 300);
        }, false);
    })();

